1. Field of the Invention
This invention relates to an improved process for preparing titanium dioxide (TiO.sub.2) pigments by continuous wet treatment, The TiO.sub.2 pigments prepared by the process of this invention can be used to make latex paint formulations having improved properties such as gloss and zinc oxide (ZnO) stability.
2. Description of the Related Art
Processes to coat TiO.sub.2 pigment with hydrous oxide coatings are well known. These processes typically involve precipitating the hydrous oxide coatings onto the pigment from salt solutions by controlling the pH of the solution, hence they are referred to as "wet treatment". Wet treatment is most frequently performed in a batch process wherein considerable residence times are needed to achieve durable coatings on the pigment.
A first step in these processes often involves preparing an aqueous slurry of TiO.sub.2 pigment particles and adjusting the pH of the slurry to about 9 or above, as described in Brand, U.S. Pat. No. 5,041,162. Brand describes a batch process for preparing a TiO.sub.2 pigment coated with silica and alumina comprising the steps of: a) adjusting the pH of the slurry to at least 9.8 and heating the slurry to 75-95.degree. C.; b) adding a soluble silicate to the slurry: c) adjusting the pH in two steps, first, rapidly adjusting the pH to 9.2-9.4 to complete deposition and cure of silica and second, rapidly adjusting the pH to 2.8-3.2; c) adding a soluble aluminate to raise the pH to 5.5-6.5 to initiate deposition of alumina coating; and continuing aluminate addition at a pH of 5.5-6.5; e) adjusting the pH to 6.5-8.5; and f) recovering the coated TiO.sub.2 pigment.
West, U.S. Pat. No. 4,125,412, provides a continuous process for wet treatment of TiO.sub.2 pigments which comprises: a) heating a TiO.sub.2 pigment slurry to 80-100.degree. C.; b) adding sodium silicate to the slurry; c) adjusting the pH to 9 to 10.5 and adjusting temperature to 85-95.degree. C. for 15-60 minutes to provide a silica coating; d) simultaneously adding sodium aluminate and acid at pH 8 to 9 to provide an alumina coating; d) curing the aluminate treated slurry for up to 30 minutes; e) adjusting the pH to 7; and e) filtering, washing and drying. Optionally pH is adjusted to 9 to 10.5 prior to the addition of the sodium silicate to the slurry in step b). However, it has been found that the pigments prepared by the West process suffer from poor zinc oxide stability and certain latex paint formulations made with such pigments have low gloss.
Brand, EP 73340 A, describes a continuous wet treatment process which involves: a) adding a soluble silicate to a TiO.sub.2 pigment slurry at a temperature of at least 85.degree. C., and a pH of 9.8 to 10.1 to form a dense silica coating; b) neutralizing the slurry in at least 3 steps by adding acid to lower the pH to 9.6-9.8, then 9.2-9.4, and finally to 3-4; c) raising the pH of the slurry to 5.0-6.5; (d) adding an alumina source to form a hydrous alumina coating; and (e) neutralizing the slurry and recovering the pigment. This patent teaches that control of pH as described is a critical element to precipitation of the silica coating.
Prior to the present invention, pigments made via continuous wet treatment processes have been found to be inferior to those made in conventional batch wet treatment processes. It is well known that continuous processes are economically more attractive than batch treatment processes. In a continuous process, treatment times can be greatly reduced and thus increase productivity without the need for capital investment. The present invention provides a continuous process that raises the level of performance of the pigment product to those of batch processes.